1. Field of the Invention
This invention is in the field of phosphors for cathode ray tubes and deals with improved material consisting of dysprosium activated calcium silicate. The phosphor is particularly useful in a cathode ray tube of high definition which is used as a terminal display apparatus for a computer, for example.
2. Description of the Prior Art
As computer technology has advanced, a picture tube of high definition which is capable of graphic display and display of Chinese characters of fine resolution has been developed for the terminal display apparatus of a computer. In such picture tubes, in order to increase the resolution, it is necessary that the horizontal scanning speed be very high, with the number of scanning lines being increased to a range from 1,000 to 2,000. At the same time, the frame repetition frequency is lowered. For this reason, when a phosphor used in an ordinary picture tube is employed, its decay time is too short and causes flickering. It is therefore necessary to use a phosphor in this type of picture tube which has a long decay time.
Some long decay phosphors have been previously suggested in the prior art. There is the green color emission phosphor, P39 (Zn.sub.2 SiO.sub.4 :Mn, As) and the red color emission phosphors, P27 (Zn.sub.3 (PO.sub.4).sub.2 :Mn), and P13 (MgSiO.sub.3 :Mn) and these have been put into practical use. However, there is no long decay phosphor which emits a white light under excitation presently known to the art.
To provide a long decay phosphor material capable of white color emission in, for example, a black and white picture tube of high definition, the phosphor P40 has been used which is produced by mixing ZnS:Ag having a blue color emission characteristic with (Zn, Cd)S:Cu having a yellowish green color emission. The P40 phosphor has the property that the (Zn, Cd)S:Cu having the yellow green color emission is excited by the blue light emitted from the ZnS:Ag having the blue color emission properties to provide a long decay emission. Since the ZnS:Ag of the blue color phosphor does not have a long decay property, a white color emission of long decay time cannot be obtained by the use of P40. Moreover, under the excitation by an electron beam, the (Zn, Cd)S:Cu having yellowish green color emission does not provide a decay time which is as long as that presented by the light excitation. Therefore, in order that the long decay property be present in the picture tube it becomes necessary to provide a phosphor screen of a particular construction which can be excited effectively by the blue color emission of ZnS:Ag, for example, a double layer coating treatment and the like. This treatment creates problems because the manufacturing process of the phosphor screen of the picture tube becomes complicated. Furthermore, even if a white color transmission phosphor is formed by mixing phosphors of long decay properties such as the above-noted P39, P27, or the like, the decay times of the respective phosphors are different so that the decay light is not a white color. At present, it has not been possible to provide phosphors which have color components whose decay characteristics are identical or substantially so. Thus, the prior art still lacks phosphors having white color emission and long decay characteristic properties.